1. Field of the Invention
The invention relates to a printing blanket for offset printing with a lithographic surface, a compressible layer, a reinforcing layer possessing low stretch characteristics, and a fabric layer on the side of the reinforcing layer, which is opposite to the lithographic printing surface.
Printing blankets which are affixed to the blanket cylinder of an offset printing machine are designed to transfer the image to be printed, normally carried on a metal printing plate, onto the material which is to be printed, normally paper, board or metal, which may be in the form of sheets or rolls. Essentially, there are three known types of printing blankets. The first type, to which the present invention is related, are so-called continuously manufactured printing blankets, which are produced continuously in the form of a roll and then cut to size according to the dimensions of the blanket cylinder upon which they are to be installed. The second and third types are individually manufactured printing blankets which are both specially produced for the blanket cylinder upon which they are to be installed and, in one case, can be slid onto the blanket cylinder like a sleeve ("sleeve printing blanket") or in the other case can be individually stretched into position using a tensioning mechanism. The second type of printing blanket has the advantage that, unlike traditional printing blankets, it does not have to be secured by means of a gap cut into the surface of the blanket cylinder (gapless cylinder). Thus, no eccentric movements are generated during rotation of the blanket cylinder and, as a result, faster revolutions and thus printing speeds are possible. However, the disadvantage of such sleeve-type printing blankets is that the printing press has to be specially designed to accommodate them and, as a consequence, is extremely expensive: the sleeve blankets themselves are also very expensive compared to existing blankets.
As mentioned above, continuously manufactured printing blankets are cut to size according to the dimensions of the blanket cylinder. In the past, mounting bars were attached to the blanket, normally by means of bolts, in order to secure it in place on the blanket cylinder. This technique is, however, obsolescent and, nowadays, disposable bars are used which are fixed onto the blanket by the simultaneous use of a press and adhesive. Once the blanket is worn out it is discarded, along with the disposable bars. Compared to the previously employed re-usable mounting bars, it is therefore important, in the case of the modern technique, that the blanket construction is such as to ensure problem-free adhesion of the disposable bars to the blanket.
2. Discussion of the Prior Art
The Japanese Patent Application No. 58-84294, Patents Abstracts of Japan, 1985 Vol. 9/No.80, describes a blanket with a lithographic surface on its topside and a metal layer on the underside. There is a compressible layer located between the metal layer and the lithographic surface. A compressible layer of this kind serves to avoid distortion caused by volume reduction in the image area and also to compensate for variations in indentation. It is an essential characteristic of the compressible layer that it does not expand laterally when compressed, i.e. that its volume actually reduces, so that no lateral distortion occurs. There are several known ways of achieving this, such as, for example the use of plastic micro-spheres suspended in a rubber compound, or the use of a microporous cellular structure with enclosed gas voids. The printing blanket described in the Japanese patent application has the great disadvantage that the underside consists of the reinforcing ply which in this case is made of metal and thus the disposable fixing bars commonly used these days to secure the blanket to the blanket cylinder cannot be reliably bonded to it. The disposable mounting bars which are made from a metal such as aluminum are difficult to bond to the metal layer and thus a safe and reliable tensioning of the blanket onto the blanket cylinder is not possible. A further disadvantage of the metal layer on the underside of the printing blanket is that the moisture which forms between the printing blanket and the blanket cylinder during the printing process, or which finds its way in there, is neither absorbed by the printing blanket nor able to find its way out laterally between printing blanket and the blanket cylinder, thus potentially causing corrosion. The same disadvantages are applicable to blankets which have on their underside a polymer or polyester film, as the disposable bars cannot easily be bonded to these materials and they also absorb no moisture.
Certain of the known printing blankets without a reinforcing layer have the disadvantage that the stabilizing layer (or several stabilizing layers) which is (are) made of woven material, typically cotton fabric, exhibits a relatively high degree of stretch. As a result, the blankets require frequent retensioning on the blanket cylinder and the print quality is adversely affected by a blurring of the individual dots. Especially in four color applications, where for the application of four colors, four successively arranged blanket cylinders are used, uneven stretching of the four successively positioned printing blankets used to print the same sheet of paper, can lead to a mis-register of the colors and thereby to a loss of image sharpness.
Furthermore, in the case of some traditional printing blankets, the stretch in the weft direction is considerably higher than that in the warp direction so that they can only be installed on the blanket cylinder with the warp direction (lengthwise) running around the cylinder. Installation of the printing blankets on the blanket cylinder in the weft direction (widthwise) is not possible, owing to the high degree of stretch.
The Japanese patent application No. 64-273158, Patents Abstracts of Japan, 1991, Vol 15/No.339 describes a blanket with a lithographic surface, a metal reinforcing layer, a compressible layer and a fabric layer which forms the underside of the printing blanket. The fabric layer on the underside of the blanket is located at the underside of the compressible layer which is bonded to the reinforcing layer on its upper side. The printing layer with the lithographic surface is attached to the metal layer by means of an undercoat or adhesive layer. The disposable bars used nowadays can, it should be said, be firmly attached to this blanket, as its underside consists of a fabric layer. However, it exhibits other serious disadvantages. The first disadvantage is that the metal reinforcing layer, consistent with its purpose of avoiding stretching of the blanket in the length or cross direction, exhibits a very low degree of stretch which, when the blanket is tensioned on the blanket cylinder, results in excessive compression of the underlying compressible layer.
This results from the fact that, during manufacture, the individual layers of the blanket are laminated together as flat surfaces, whereas during the printing process the blanket is installed upon the blanket cylinder in the form of a part-circle. As the compressible layer is situated beneath the metal reinforcing layer, it has a shorter radius than the metal layer. Consequently, when the blanket is tensioned, owing to the low stretch characteristics of the metal layer, the compressible layer is compressed and is thus unable or severely limited in its ability to fulfill its intended purpose during the printing process, namely the avoidance of distortion and compensation for variations in indentation, which renders it useless.
Furthermore, owing to the gauge loss of this blanket, only limited printing is possible, since, for instance in the case of blanket to blanket web offset printing, no adjustment is possible.
A further disadvantage of the type of printing blanket described is the fact that it cannot be used for varnishing purposes. In varnishing applications, areas of the printing surface are as a rule cut out (stripped) - either by hand or with the aid of a CAD machine--which correspond to those areas of the printing substrate which are not to be varnished. The remaining unstripped areas of the lithographic surface take up the varnish and print it onto the corresponding areas of the printing substrate which are to be varnished. The quality of the varnish applied and in particular also the life of the printing blanket employed are determined by the depth of the cut-outs. The deeper the cut-out, the easier it is to avoid the accumulation of varnish in the stripped areas which can then be transferred to the printed item (substrate), leading to reduced quality and consequent machine down-time. In the case of the blanket described in the above Japanese patent application, however, the metal layer is located directly beneath the blanket face layer which forms the lithographic surface. If used for varnishing therefore, the maximum depth of the cut-out cut into the face can only be equivalent to the thickness of the face, since it is only possible to cut out to the hard metallic layer. The blanket described is thus poorly suited to use for varnishing applications, since the depth of the cut out is limited by the face thickness and is thus too small.
The cut out areas of the blanket are too shallow and, even after a short print-run, this leads to an accumulation of varnish in the stripped out areas. The blanket described here is therefore unsuitable for varnishing work over extended periods.